Silicon-starved nitride spacer deposition

ABSTRACT

Bridging between a metal silicide e.g., nickel silicide, layer on a gate electrode and metal silicide layers on associated source/drain regions is avoided by forming silicon-starved silicon nitride sidewall spacers having substantially no or significantly reduced Si available for reaction with deposited metal, e.g., nickel.

FIELD OF THE INVENTION

The present invention relates to semiconductor device fabrication,particularly to self-aligned silicide (salicide) technology.

BACKGROUND ART

As gate electrode lengths are scaled down, the source and drainjunctions and polycrystalline silicon line width must also be scaleddown. However, scaling down the source and drain junctions andpolycrystalline line width increases parasitic resistance in the sourceand drain diffusion layers and the gate electrode, and also increasesthe sheet and contact resistance of the gate electrode and source/drainregions.

Salicide technology comprises forming metal silicide layers on thesource/drain regions and/or on the gate electrode of a semiconductorwafer in a self-aligned manner. A conventional approach to reduceresistivity involves forming a multi-layered structure comprising a lowresistance refractory metal silicide layer on a doped polycrystallinesilicon, typically referred to as a polycide. Salicide technologyreduces parasitic sheet and contact resistance in the source and draindiffusion layers and the gate electrode that results from scaling downthe source and drain junctions and polycrystalline silicon line width.

Silicides are typically formed by reacting a metal with silicon (Si)within a specified temperature range for a specific period of time.Silicide layers may be self-aligned by different techniques. Forexample, the metal can be selectively deposited on the gate electrodeand on the source/drain regions, with subsequent annealing to react themetal with underlying Si in the source/drain regions and the gateelectrode to form the metal silicide layers. Alternatively, sidewallspacers, e.g., silicon nitride or silicon dioxide, are formed on theside surfaces of the gate electrode, followed by a blanket deposition ofmetal and annealing to react the metal with Si in the gate electrode andthe source/drain regions, while the sidewall spacers prevent metalreaction with Si from the side surfaces of the gate electrode.

During annealing, the wafer is heated to a reaction temperature and heldat the reaction temperature for a period of time sufficient for themetal layer to react with underlying Si to form a metal silicide layeron the source/drain regions and the gate electrode. Multiple annealingsteps may be employed.

Various metals react with Si to form a metal silicide; however, titanium(Ti) and cobalt (Co) are currently the most common metals used to createmetal silicides when manufacturing semiconductor devices utilizingsalicide technology. Recently, attention has turned towards nickel (Ni)to form nickel silicide utilizing salicide technology. Nickel silicideavoids many limitations associated with TiSi₂ and CoSi₂. Unlike Ti whereSi diffuses into the metal layer when forming a Ti silicide, Ni, likeCo, diffuses into Si, which helps to limit bridging between the metalsilicide layer in the gate electrode and the metal silicide layer on theassociated source/drain regions. The formation of nickel siliciderequires less Si than TiSi₂ and CoSi₂. Nickel silicide also exhibitsalmost no line width dependence on sheet resistance. Nickel silicide isnormally annealed in a one step process, vis-à-vis a process requiringan anneal, an etch, and a second anneal, as occurs in TiSi₂ and CoSi₂saliciding. In addition, nickel silicide exhibits lower film stress,i.e., causes less wafer distortion, than conventional Ti or Co suicides.

Salicide processing efficiency is improved through the use of sidewallspacers. Sidewall spacers allow a blanket layer of metal to be depositedover the wafer surface. Sidewall spacers typically comprise silicondioxide or silicon nitride, but silicon nitride sidewall spacers areoften preferable because silicon nitride is highly conformal and thesidewall spacers can be added and removed as needed throughout themanufacturing process. However, the use of silicon nitride sidewallspacers with salicide technology results in bridging between the metalsilicide layer on the gate electrode and the metal silicide layers onassociated source/drain regions, particularly when Ni is used.

There is a need for salicide technology that avoids bridging between themetal silicide layer on the gate electrode and the metal silicide layerson the source/drain regions when using silicon nitride sidewall spacers,particularly when forming nickel salicide.

DISCLOSURE OF THE INVENTION

These and other needs are satisfied by embodiments of the presentinvention, which include a method of manufacturing a semiconductordevice without bridging between a metal silicide layer, particularlynickel silicide, on the gate electrode and metal silicide layers, e.g.,nickel silicide layers, on the source/drain regions usingsilicon-starved silicon nitride sidewall spacers. The method comprisessupplying silane (SiH₄) at a flow rate of approximately 100 sccm toapproximately 250 sccm, supplying nitrogen (N₂) at a flow rate ofapproximately 3,000 sccm to approximately 6,000 sccm, supplying ammonia(NH₃) at a flow rate of approximately 2,000 sccm to approximately 4,000sccm, applying radio frequency power of approximately 200 watts toapproximately 350 watts, applying a pressure of approximately 1.6 torrto approximately 2.2 torr, and maintaining the temperature atapproximately 360° C. to approximately 380° C.

The present invention advantageously limits metal bonding with Si in thesilicon nitride sidewall spacers by reducing the availability of free Siin the sidewall spacers. Part of the silicidation process involvesforming a metal silicide layer e.g., NiSi, on the polysilicon gateelectrode and associated source/drain regions using salicide technologywithout bridging therebetween. In accordance with embodiments of thepresent invention, bridging is avoided by forming silicon-starvedsilicon nitride sidewall spacers, whereby the reduced Si in the siliconnitride sidewall spacers eliminates the free Si available to react withthe metal subsequently deposited to form the metal silicide layer.

Other advantages of the present invention will become readily apparentto those skilled in the art from the following detailed description. Theembodiments shown and described provide illustration of the best modecontemplated for carrying out the invention. The invention is capable ofmodifications in various obvious respects, all without departing fromthe invention. Accordingly, the drawings are to be regarded asillustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the attached drawings, wherein elements having thesame reference numeral designations represent like elements throughout.

FIGS. 1-3 schematically illustrate sequential phases of a prior artsalicide technique resulting in bridging.

FIGS. 4-7 schematically illustrate sequential phases of a method inaccordance with embodiments of the present invention without bridgingbetween the metal silicide layer on the gate electrode and the metalsilicide layers on the associated source/drain regions.

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses and solves problems related to forming ametal silicide layer on the gate electrode and source/drain regions of asemiconductor wafer. As device geometries shrink into the deepsub-micron regime, metal silicide bridging occurs along the surface ofsilicon nitride sidewall spacers between the metal silicide layer on thegate electrode and metal silicide layers on associated source/drainregions. For example, adverting to FIG. 1, in attempting to implementnickel silicide technology utilizing silicon nitride sidewall spacers, agate electrode 2 is formed on silicon substrate 4 with a gate insulatinglayer 6 therebetween. An oxide liner 7 is disposed as a buffer layer onthe side surfaces of the gate electrode 2. A conformal layer of siliconnitride is then deposited followed by anisotropic etching to formsilicon nitride sidewall spacers 8 on opposing side surfaces of gateelectrode 2. After anisotropic etching, contaminants such as etchingresidues are removed by wet cleaning. A thin oxide layer, as at athickness of about 10 Å to about 15 Å, is formed on the exposed surfacesof silicon nitride sidewall spacers 8, on the exposed upper surface ofgate electrode 2 and on the exposed surface of substrate 4, acting as abarrier layer preventing nickel silicidation. Sputter etching in argonis then conducted to remove the thin oxide layer from the exposedsurfaces of the silicon nitride sidewall spacers 8, from the uppersurface of the gate electrode 2 and from the exposed surface ofsubstrate 4. Shallow source/drain extensions 10 and source/drain regions12 are formed in a conventional manner.

Adverting to FIG. 2, a Ni layer 20 is deposited over the wafer surface.The wafer is then subject to annealing to react Ni with the underlyingSi.

As shown in FIG. 3, following heating, a nickel silicide layer 30 isformed on the upper surface of gate electrode 2 and a layer of nickelsilicide 32 is formed on associated source/drain regions 12. However, itwas found that a thin layer of nickel silicide 34, as at a thickness ofabout 30 Å to 60 Å, is undesirably formed along the exposed surfaces ofsilicon nitride sidewall spacers 8 causing bridging and, hence, shortingbetween nickel silicide layer 30 and nickel silicide layers 32.

After considerable experimentation and investigation, it was postulatedthat the problem of nickel silicide formation 34 along the siliconnitride sidewall spacers 8 stemmed from the reaction of Ni with danglingSi bonds in the silicon nitride sidewall spacers 8. The presentinvention addresses and solves such problems by reducing free Si insilicon nitride sidewall spacers 8.

In accordance with embodiments of the present invention, silicon-starvedsilicon nitride sidewall spacers are formed on the wafer. It was foundthat decreasing silane flow during spacer formation eliminates free Sileaving substantially no or a significantly reduced amount of Siavailable for reaction with the deposited metal, e.g., Ni, therebyavoiding metal silicide bridging, such as that denoted by referencenumeral 34 in FIG. 3.

An embodiment of the present invention is schematically illustrated inFIGS. 4 through 7, wherein similar reference numerals denote similarfeatures. Adverting to FIG. 4, a gate electrode 40, e.g., dopedpolycrystalline silicon, is formed on substrate 42, which can be n-typeor p-type doped silicon, with a gate insulating layer 44 therebetween.Gate insulating layer 44 is typically silicon dioxide formed by thermaloxidation or chemical vapor deposition (CVD). A spacer oxide liner 46 isdisposed on the opposing side surfaces of gate electrode 40 as a bufferbetween the silicon-starved silicon nitride layer 48 and the sidesurfaces of the gate electrode 40.

Subsequent to forming spacer oxide liner 46, a silicon-starved siliconnitride layer 48 is deposited to form silicon-starved silicon nitridesidewall spacers. According to an embodiment of the present invention,silicon-starved silicon nitride sidewall spacers are formed by supplyingSiH₄ at a flow rate of about 100 sccm to about 250 sccm, such as about150 sccm to about 200 sccm, e.g., about 175 sccm. N₂ is supplied at aflow rate of about 3,000 sccm to about 6,000 sccm, or 4,000 sccm toabout 5,000 sccm, e.g., at about 4,500 sccm. NH₃ is supplied at a flowrate of about 2,000 sccm to about 4,000 sccm, or about 2,500 sccm toabout 3,500 sccm, e.g., 3,000 sccm. A radio frequency power supply isapplied at about 200 watts to about 350 watts, such as about 250 wattsto about 300 watts, e.g., about 275 watts. Pressure is applied at about1.6 torr to about 2.2 torr, such as about 1.8 torr to about 2.0 torr,e.g., 1.9 torr. The temperature is maintained at about 360° C. to about380° C., such as about 365° C. to about 375° C., e.g., about 370° C.

As depicted in FIG. 5, the silicon-starved silicon nitride layer 48formed in accordance with an embodiment of the present invention isetched using conventional methods to form silicon-starved nitridesidewall spacers 50.

Turning to FIG. 6, a blanket layer of a refractory metal is deposited,as by sputtering or other method, on the gate electrode 40, thesource/drain regions 12 and the silicon-starved silicon nitride sidewallspacers 50. The refractory metal can comprise cobalt (Co), titanium(Ti), tungsten (W), tantalum (Ta) or nickel (Ni).

As illustrated in FIG. 7, once the refractory metal is deposited,heating is conducted, as by rapid thermal annealing, to react therefractory metal with the underlying Si to form a metal silicide layer60 on the polysilicon gate electrode 40 and metal silicide layers 62 onthe source/drain regions 12. When Ni is the refractory metal used, rapidthermal annealing, as at a temperature of approximately 300° C. toapproximately 450° C., is conducted during which a low resistivity metalsilicide layer 60, e.g., NiSi, is formed on gate electrode 40 whilenickel silicide layers 62 are formed on source/drain regions 12 adjacentto the silicon-starved silicon nitride sidewall spacers 50. Additionalannealing steps may be employed when using Co or Ti to form a metalsilicide, e.g., TiSi₂ or CoSi₂.

Unreacted metal on the surfaces of the silicon-starved silicon nitridesidewall spacers 50 is then easily removed, as by wet chemicalstripping. In an embodiment of the present invention, the unreactedrefractory metal is removed by immersing the wafer into a solution ofH₂SO₄, H₂O₂ and water (SPM) or a solution of NH₄OH, H₂O₂ and water(APM).

By forming silicon-starved silicon nitride sidewall spacers 50 havingsubstantially no or significantly reduced free Si, reaction of the metallayer 52 with Si in the silicon-starved silicon nitride sidewall spacers50 is effectively prevented. Accordingly, the inventive methodologyavoids the formation of metal silicide on the silicon-starved siliconnitride sidewall spacers 50 and, hence, prevents bridging between themetal silicide layer 60 on the upper surface of gate electrode 40 andmetal silicide layers 62 on the source/drain regions 12 adjacent to thesilicon-starved silicon nitride sidewall spacers 50.

The present invention, therefore, enables the implementation of metalsilicide methodology, advantageously utilizing silicon-starved siliconnitride sidewall spacers 50 without bridging between the metal silicidelayer 60 formed on the upper surface of the gate electrode 40 and themetal silicide layers 62 formed on associated source/drain regions 12.

Another aspect of the present invention relates to a semiconductordevice that includes a polysilicon gate electrode 40, source/drainregions 12, and silicon nitride sidewall spacers 50, wherein the siliconnitride sidewall spacers 50 are silicon-starved. Metal silicide layers60 and 62 are formed by depositing a refractory metal, e.g., Co, Ti, Ta,Ni or W, on the wafer and heating to react the refractory metal with Si.Unreacted refractory metal is removed by wet chemical stripping.

The present invention enjoys industrial applicability in fabricating anyof various types of semiconductor devices. The present invention hasparticular applicability in semiconductor devices with high circuitspeeds having design features in the deep sub-micron regime.

Only the preferred embodiment of the invention and but a few examples ofits versatility are shown and described in the present disclosure. It isto be understood that the invention is capable of use in various othercombinations and environments and is capable of changes or modificationswithin the scope of the inventive concept as expressed herein.Well-known processing structures have not been described in detail inorder not to unnecessarily obscure the present invention.

What is claimed is:
 1. A method of manufacturing a semiconductor device,the method comprising the steps of: forming a silicon gate electrode,having an upper surface and side surfaces, overlying a silicon substratewith a gate dielectric layer therebetween, a spacer oxide liner disposedon the side surfaces, and source/drain regions in the substrate:depositing a silicon nitride layer at: a SiH₄ flow rate of approximately100 sccm to approximately 250 sccm; a N₂ flow rate of approximately3,000 sccm to approximately 6,000 sccm; a NH₃ flow rate of approximately2,000 sccm to approximately 4,000 sccm; a radio frequency power ofapproximately 200 watts to approximately 350 watts; a pressure ofapproximately 1.6 torr to approximately 2.2 torr; and a temperaturemaintained at about 360° C. to approximately 380° C.; anisotropicallyetching the silicon nitride layer to form silicon nitride sidewallspacers on the spacer oxide liner; depositing a refractory metal layer;heating to react the refractory metal with underlying Si to form a metalsilicide layer on the gate electrode and a metal silicide layer on thesource/drain regions; and removing unreacted refractory metal from thesilicon nitride sidewall spacers.
 2. The method of claim 1, comprisingdepositing the silicon nitride layer using plasma enhanced chemicalvapor deposition (PECVD).
 3. The method of claim 1, comprising supplyingSiH₄ at a flow rate of approximately 150 sccm to approximately 200 sccm.4. The method of claim 3, comprising supplying SiH₄ at a flow rate ofapproximately 175 sccm.
 5. The method of claim 1, comprising supplyingN₂ at a flow rate of approximately 4,000 sccm to approximately 5,000sccm.
 6. The method of claim 5, comprising supplying N₂ at a flow rateof approximately 4,500 sccm.
 7. The method of claim 1, comprisingsupplying NH₃ at a flow rate of approximately 2,500 sccm toapproximately 3,500 sccm.
 8. The method of claim 7, comprising supplyingNH₃ at a flow rate of approximately 3,000 sccm.
 9. The method of claim1, comprising depositing the silicon nitride layer at a radio frequencypower of approximately 250 watts to approximately 300 watts.
 10. Themethod of claim 9, comprising depositing the silicon nitride layer at aradio frequency power of approximately 275 watts.
 11. The method ofclaim 1, comprising depositing the silicon nitride layer at a pressureof approximately 1.8 torr to approximately 2.0 torr.
 12. The method ofclaim 11, comprising depositing the silicon nitride layer at a pressureof approximately 1.9 torr.
 13. The method of claim 1, comprisingdepositing the silicon nitride layer at a temperature maintained atapproximately 365° C. to approximately 375° C.
 14. The method of claim13, comprising depositing the silicon nitride layer at a temperaturemaintained at approximately 370° C.
 15. The method of claim 1, whereinthe refractory metal is selected from the group consisting of cobalt,titanium, tungsten, tantalum and nickel.
 16. The method of claim 15,wherein the refractory metal comprises nickel.
 17. The method of claim1, wherein the metal silicide layer comprises NiSi.
 18. The method ofclaim 1, comprising removing the unreacted refractory metal by wetchemical stripping.
 19. The method of claim 18, comprising wet chemicalstripping by immersing the substrate in a solution of NH₄OH, H₂O₂ andwater (APM) or immersing the substrate in a solution of H₂SO₄, H₂O₂ andwater (SPM).